The standardization of a Long Term Evolution (LTE) system which is a wireless communication system of 3.9G mobile phones has been completed, and the standardization of LTE-Advanced (LTE-A) (also referred to as IMT-A) system which is an enhancement of the LTE system as one of 4G wireless communication systems is being currently performed.
In the LTE-A system (from LTE Rel. 10 on), a carrier aggregation (CA) technology of using one system band of the LTE system as a component carrier (CC) (also referred to as a serving cell) and simultaneously using a plurality of CCs has been adopted. In a case where CA is performed, one CC is used as a primary cell (Pcell) capable of realizing all functions, and other CCs are used as a secondary cell (Scell).
Reserving frequency resources while coping with a rapid increase in data traffic in the LTE system is an important issue. A frequency band assumed for the LTE system so far has been a frequency band referred to as a so-called licensed band permitted for use by a country or a locality in which a radio communication operator provides services, and has limitations in an available frequency band.
Thus, the provision of an LTE system that uses a frequency band referred to as a so-called unlicensed band which does not require permission for use by a country or a locality has been recently discussed (see NPL 1). The CA technology adopted by the LTE-A system is also applied to an unlicensed band, and thus, it is expected that the LTE-A system will be able to cope with a rapid increase in data traffic with high efficiency.